Use valgrind.h to query whether the test suite is run in valgrind

Currently the test suite sets the USING_VALGRING=1 environment variable. That's not automatically the case when run manually though, causing some confusing errors. The valgrind documentation describes what's available to programs, basically seems to down to

#include <valgrind.h>if (RUNNING_ON_VALGRIND) { // do stuff}

Also noteworthy from the valgrind docs linked above:

You are encouraged to copy the valgrind/*.h headers into your project's include directory, so your program doesn't have a
compile-time dependency on Valgrind being installed. The Valgrind headers, unlike most of the rest of the code, are under
a BSD-style license so you may include them without worrying about license incompatibility.